here is the workaround I used for the mean time! may help others.
- Install .net sdk 2.x (using .NET Core Tool Installer)
- Prepare Analysis Configuration
- Install .net sdk 3.x (using .NET Core Tool Installer)
- Build your application (which is targeting netcore3.x)
- Test your application (which is targeting netcore3.x)
- Install .net sdk 2.x (using .NET Core Tool Installer)
- Run Code Analysis
- Publish Quality Gate Result
verify in SonarQube project the analysis as part of build is posted.